Reducing Moisture Absorption in Polypropylene Nanocomposites for Automotive Headlamps Using Hydrophobicity-Modified Graphene/Montmorillonite

Polypropylene (PP) is used as a housing material in automotive headlamps but can cause fogging as a result of absorbed moisture and temperature differences between the exterior and interior of the housing. In this study, PP was combined with a graphene/montmorillonite hybrid (MMT-G) to yield a nanocomposite with reduced moisture absorption. Crucially, the modified nanofiller had low hydrophilicity and good compatibility with the PP matrix. Notably, the water contact angle of the MMT-G improved by 676%. Furthermore, the maximum moisture absorption of the PP/MMT-G nanocomposites was reduced by up to 11.22% compared to that of commercial PP composites, and the weight of the headlamp housing was decreased by 3.6%. Therefore, the designed nanocomposites are expected to help mitigate headlamp fogging while slightly reducing the housing weight.


Introduction
Automotive headlamps are key automotive components, particularly when forward visibility is poor. In the past decades, headlamps served as simple light sources; however, in recent years, their functions have diversified because of the increasing importance of vehicle appearance and higher safety demands. Accordingly, the internal geometry of the lamp is becoming more complex, and headlamp parts are increasingly prepared using polymers because of their high design freedom [1][2][3][4]. However, because of the added functions and more complex design, automotive headlamps have gradually become heavier. Therefore, strategies to decrease the weight and increase the rigidity of such parts should be developed. Further, the internal temperature increases and decreases when the lamp, which is enclosed, is turned on and off, respectively, which results in the release of the moisture contained in the polymeric housing and increases the humidity inside the lamp. Consequently, fogging occurs because of the temperature difference between the inside and outside of the housing, which causes performance deterioration and negatively affects appearance.
Several studies have analyzed the aforementioned fogging phenomenon. For example, Guzej and Zachar [2] used computational fluid dynamics (CFD) simulations with a simplified three-dimensional (3D) model of a headlamp to simulate phenomena related to fluid flow and phase changes. Watson et al. [1] described a technique for the transient numerical simulation of liquid film formation on surfaces during lighting, as well as thermal analysis, using CFD, including the effects of the properties of the liquid film on thermal solution. Singh et al. [3] implemented the Eulerian wall film (EWF) approach in a three-dimensional

Nanofiller Preparation
Before the preparation of a graphene/montmorillonite hybrid (MMT-G), MMT was first treated with organic agents. Specifically, organo-MMT was synthesized through a cation exchange reaction with DDAC. Briefly, 10 mL of HCl was added to 500 mL of deionized (DI) water and stirred well. Thereafter, 8 g of DDAC was dissolved in this solution, which was then stirred at 80 • C for 3 h. Meanwhile, 29.09 g of MMT-Ca 2+ was added to 1300 mL of DI water and stirred vigorously at 80 • C for 1 h. Afterward, the resulting mixture was mixed with a pre-prepared DDAC solution and allowed to react vigorously at 80 • C for 12 h. The synthesized organo-MMT was centrifuged, washed several times with a mixed solvent of dimethylacetamide/DI water, washed again with DI water, and freeze-dried for 48 h to obtain organo-MMT.
Second, positively charged graphene (CMG + ) capable of forming chemical interactions with the cationic nitrogen (N + ) of organo-MMT was produced. For CMG + , GO was prepared using an improved method [18] reported previously [17,19]. Subsequently, a CMG + colloid with high dispersibility was prepared with 4,4-oxidianiline. Thereafter, 1000 mL of a 0.1% GO aqueous solution was treated using an ultrasonic homogenizer for at least 2 h, and a solution containing 40 g of 4,4-oxidianiline in 1000 mL of DMF was prepared, mixed, and stirred at 90 • C for 40 h. After the reactant was cooled to room temperature, the precipitate was obtained using centrifugation, washed seven to eight times with acetone, filtered to obtain CMG + (positively charged graphene), and then immediately suspended in ethanol. As a result, an electrostatic repulsive force was generated between the graphene monolayers owing to the positive charge (-N + ) of the substituted 4,4-oxidianiline, and a highly dispersed functional graphene colloid was obtained.
Finally, organo-MMT was added to ethanol and stirred for 30 min with a homogenizer to prepare a 5 wt% dispersion. The CMG + colloid was then further modified by the addition of different amounts of graphene (0.1 wt% (MMT-0.1G), 0.5 wt% (MMT-0.5G), 1.0 wt% (MMT-1.0G), and 3.0 wt% (MMT-3.0G)) and stirred with a homogenizer. During this process, adsorption via self-assembly occurred via electrostatic attraction. After the adsorption process, the coprecipitated materials were filtered under reduced pressure, separated, washed with ethanol, and dried for 24 h in an oven at 60 • C to obtain the MMT-G nano-hybrids.

Nanocomposite Preparation
Nanocomposites were fabricated using a extruder (twin screw type, 32 mm, L/D = 40/1, Unee Plus Co., Hwaseong, Republic of Korea). The temperatures of the twin screw extruder were set to 210 and 200 • C at the feeder and die hole, respectively, and the rotor speed was set to 400 revolutions per minute (rpm). First, polypropylene was supplied from the main feeder at 30 kg h −1 , and the supply from the side feeder was such that the talc contents were 30 (reference sample) and 25 wt% (base sample). The PP/MMT-G nanocomposites were prepared by setting the base sample to be supplied at 30 kg h −1 from the main feeder, and that from the side feeder was supplied such that the MMT-G contents were 0.2, 0.3, 0.5, and 1.0 wt%.

Characterization and Instruments
The nanofillers were analyzed using scanning electron microscopy (SEM, Helios 5 Hydra CX, Thermo Fisher Scientific, MA, USA), Raman spectroscopy (Confotec MR350, SOL Instruments, Minsk, Belarus), zeta potential measurements (LITESIZER 500, Anton Paar, Graz, Austria), and Fourier transform infrared (FT-IR) spectroscopy (Spectrum Two, PerkinElmer, Waltham, MA, USA). To obtain the interlayer spacing of graphene and MMT-based fillers modified via different surface treatments, X-ray diffraction (XRD, smallangle X-ray scattering (SAXS) type, XEUSS 3.0 COMPACT, Xenocs, Grenoble, France) was performed on the MMT-G samples. The SAXS apparatus used Cu-K α radiation (λ = 1.54 Å) generated at 50 kV and 0.6 mA. The data were collected between 2 • and 10 • in the 2θ mode. The water contact angle measurements were performed using a Phoenix-MT(T) system (Surface and Electro-Optics, Suwon, Republic of Korea) under ambient conditions. The mechanical properties (tensile properties (strength and elongation) and flexural properties (strength and modulus)), melt flow index, and Izod impact strength of the nanocomposites were measured using a universal testing machine (UT-100F, MTDI Korea Co., Daejeon, Republic of Korea), QM280 (Qmesys, Uiwang, Republic of Korea), and QM700L (Qmesys, Uiwang, Republic of Korea). The moisture absorption of the PP nanocomposites was calculated by measuring the weights before and after moisture absorption after leaving them in a constant temperature and humidity chamber (TH-G-180, Jeio Tech, Daejeon, Republic of Korea) at 80 • C and 85% relative humidity for 20,000 h.
Headlamp housings were also manufactured using the PP nanocomposites. The housing was shaped on a mold using a simplified form of the conventional mold and formed through an injection process. A 1050 t injection machine (LGH-1050M, LS Mtron, Anyang, Republic of Korea) was used, and the injection conditions were an injection temperature of 235-300 • C, mold temperature of 80 • C, injection pressure of 9 MPa, injection time of 4.3 s, holding pressure of 7 MPa, holding time of 8.0 s, and cooling time of 35.0 s. The molding characteristics were compared by analyzing the error relative to the design shape using a 3D measuring instrument (ATOS 2 Triple, GOM, Braunschweig, Germany).

Nanofiller Characterization
MMT-Ca 2+ is a type of smectite and silicate-layered clay mineral. In its structure, an alumina octahedral sheet is located between the silica tetrahedral sheets, and oxygen atoms are located between the silica tetrahedral sheets and alumina octahedral sheets [20,21]. MMT-Ca 2+ achieves charge neutrality by intercalating Ca 2+ cations between layers. Therefore, we designed MMT-G in which a graphene sheet was bonded to the silicate surface after the cations intercalated between the layers had been organically modified via ion exchange treatment. After the modification of these interlayers, the hydrophilicity of the interlayer structure is reduced, and the affinity for PP is increased. As a result, PP can be intercalated into the interlayer during compounding with the PP matrix. Moreover, the addition of hydrophilic MMT to a hydrophobic PP matrix is expected to increase the moisture absorption rate [12]. Therefore, as shown in Figure 1a, the organo-MMT surface was coated with positively charged graphene (CMG + ) to reduce the hydrophilicity of the MMT and improve the mechanical properties. In addition, the graphene was bonded to the surface of the silicate, and the surface became hydrophobic, leading to a high affinity of both the interlayers and surfaces toward PP molecules. be intercalated into the interlayer during compounding with the PP matrix. Moreover, the addition of hydrophilic MMT to a hydrophobic PP matrix is expected to increase the moisture absorption rate [12]. Therefore, as shown in Figure 1a, the organo-MMT surface was coated with positively charged graphene (CMG + ) to reduce the hydrophilicity of the MMT and improve the mechanical properties. In addition, the graphene was bonded to the surface of the silicate, and the surface became hydrophobic, leading to a high affinity of both the interlayers and surfaces toward PP molecules. Figure 1b-d show SEM images of various MMT samples. As shown in Figure 1b, MMT-Ca 2+ is agglomerated without delamination. However, in the case of organo-MMT (Figure 1c), less aggregation is observed compared with MMT-Ca 2+ , which is expected to be intercalated or exfoliated when combined with PP. Furthermore, Figure 1d confirms that MMT maintains its shape without aggregation in the graphene hybrid. Based on a comparison with Figure 1c,d, the surface of organo-MMT is rough, whereas the surface of MMT-G is smooth because of the coating with graphene sheets.  Figure 2 shows the Raman spectra of GO and CMG + , which are used to assess the structural features of the functionalized graphene used to fabricate MMT-G. Crucially, Raman spectroscopy is considerably useful for the analysis of graphene-containing materials. In particular, the disorder (D) peak (1360 cm −1 ) represents defects in the sp 2 crystal structure of graphene, whereas the graphite (G) peak (1580 cm −1 ) represents the ordered (non-defective) C=C bonds and arises because of the vibrations of neighboring atoms in the hexagonal structure in the opposite direction [19]. Figure 2 shows the Raman spectra of GO and CMG + , and characteristic D and G bands are visible. Further, the intensity of the D band is high, probably as a result of functionalization. The D/G intensity ratio of GO   Figure 1c), less aggregation is observed compared with MMT-Ca 2+ , which is expected to be intercalated or exfoliated when combined with PP. Furthermore, Figure 1d confirms that MMT maintains its shape without aggregation in the graphene hybrid. Based on a comparison with Figure 1c,d, the surface of organo-MMT is rough, whereas the surface of MMT-G is smooth because of the coating with graphene sheets. Figure 2 shows the Raman spectra of GO and CMG + , which are used to assess the structural features of the functionalized graphene used to fabricate MMT-G. Crucially, Raman spectroscopy is considerably useful for the analysis of graphene-containing materials. In particular, the disorder (D) peak (1360 cm −1 ) represents defects in the sp 2 crystal structure of graphene, whereas the graphite (G) peak (1580 cm −1 ) represents the ordered (non-defective) C=C bonds and arises because of the vibrations of neighboring atoms in the hexagonal structure in the opposite direction [19]. Figure 2 shows the Raman spectra of GO and CMG + , and characteristic D and G bands are visible. Further, the intensity of the D band is high, probably as a result of functionalization. The D/G intensity ratio of GO is 1.16, whereas that of functionalized graphene (CMG + ) is calculated to be 0.96, indicating that the structural defects are reduced via reduction and functionalization. Figure 3 shows the zeta potential analysis results for CMG + and organo-MMT. The sign and magnitude of the zeta potential provide information regarding the surface charge of a particle [19]. As shown in Figure 3, CMG + has a strong surface positive charge (+60.2 mV) and, hence, adsorbs to organo-MMT (−35.0 mV), which has a negative charge on the silicate surface, via electrostatic self-assembly. In particular, zeta potentials higher than 30 mV are Nanomaterials 2023, 13, 1439 6 of 12 indicative of highly dispersible particles [19]. Therefore, based on its high zeta potential, CMG + has a substantially high dispersion stability and a graphene structure comprising a monolayer and graphene with a few layers.   Figure 3 shows the zeta potential analysis results for CMG + and organo-MMT. The sign and magnitude of the zeta potential provide information regarding the surface charge of a particle [19]. As shown in Figure 3, CMG + has a strong surface positive charge (+60.2 mV) and, hence, adsorbs to organo-MMT (−35.0 mV), which has a negative charge on the silicate surface, via electrostatic self-assembly. In particular, zeta potentials higher than 30 mV are indicative of highly dispersible particles [19]. Therefore, based on its high zeta potential, CMG + has a substantially high dispersion stability and a graphene structure comprising a monolayer and graphene with a few layers.   is 1.16, whereas that of functionalized graphene (CMG + ) is calculated to be 0.96, indicating that the structural defects are reduced via reduction and functionalization.  Figure 3 shows the zeta potential analysis results for CMG + and organo-MMT. The sign and magnitude of the zeta potential provide information regarding the surface charge of a particle [19]. As shown in Figure 3, CMG + has a strong surface positive charge (+60.2 mV) and, hence, adsorbs to organo-MMT (−35.0 mV), which has a negative charge on the silicate surface, via electrostatic self-assembly. In particular, zeta potentials higher than 30 mV are indicative of highly dispersible particles [19]. Therefore, based on its high zeta potential, CMG + has a substantially high dispersion stability and a graphene structure comprising a monolayer and graphene with a few layers.   (Figure 4d) [22][23][24]. In Figure 4a, bands corresponding to the stretching and bending of the -OH groups of adsorbed water are present at approximately 3430 cm −1 and 1630 cm −1 , respectively. In contrast, these bands are significantly weak in the spectrum of organo-MMT (Figure 4b) because of the drying process used during MMT modification [25].   (Figure 4d) [22][23][24]. In Figure 4a, bands corresponding to the stretching and bending of the -OH groups of adsorbed water are present at approximately 3430 cm −1 and 1630 cm −1 , respectively. In contrast, these bands are significantly weak in the spectrum of organo-MMT (Figure 4b) because of the drying process used during MMT modification [25]. Figure 5 shows the XRD patterns of samples with different degrees of MMT functionalization used to investigate the interlayer structure of the MMT-based nanofiller. The distance between the layers of nanofiller was calculated using Bragg's law [17,26]. As shown in Figure 5a, the d-spacing of organo-MMT was determined to be 2.49 nm, indicating an increase from that of MMT-Ca 2+ (1.23 nm). Furthermore, the d-spacing of MMT-G was calculated to be 2.49 nm, which is similar to that of organo-MMT. Notably, the increase in the d-spacing of the nanofiller facilitated even dispersion in the PP matrix. The XRD patterns of GO and CMG + are shown in Figure 5b, revealing graphene interlayer distances of 9.601 and 3.759 Å for GO and CMG + , respectively. Because graphene is obtained through the exfoliation of graphite, they have identical crystal structures, suggesting a layer spacing Nanomaterials 2023, 13, 1439 7 of 12 of 3.36 Å. Further, the layer spacing of GO increased owing to oxidation, whereas that of CMG + narrowed again because of the small number of peripheral defects as a result of reduction and functionalization. In particular, the layer spacing of CMG + was wider than that of graphite because of the introduction of functional groups, and the diffraction peak of CMG + was also broad.  Figure 5 shows the XRD patterns of samples with different degrees of MMT functionalization used to investigate the interlayer structure of the MMT-based nanofiller. The distance between the layers of nanofiller was calculated using Bragg's law [17,26]. As shown in Figure 5a, the d-spacing of organo-MMT was determined to be 2.49 nm, indicating an increase from that of MMT-Ca 2+ (1.23 nm). Furthermore, the d-spacing of MMT-G was calculated to be 2.49 nm, which is similar to that of organo-MMT. Notably, the increase in the d-spacing of the nanofiller facilitated even dispersion in the PP matrix. The XRD patterns of GO and CMG + are shown in Figure 5b, revealing graphene interlayer distances of 9.601 and 3.759 Å for GO and CMG + , respectively. Because graphene is obtained through the exfoliation of graphite, they have identical crystal structures, suggesting a layer spacing of 3.36 Å . Further, the layer spacing of GO increased owing to oxidation, whereas that of CMG + narrowed again because of the small number of peripheral defects as a result of reduction and functionalization. In particular, the layer spacing of CMG + was wider than that of graphite because of the introduction of functional groups, and the diffraction peak of CMG + was also broad.     Figure 5 shows the XRD patterns of samples with different degrees of MMT functionalization used to investigate the interlayer structure of the MMT-based nanofiller. The distance between the layers of nanofiller was calculated using Bragg's law [17,26]. As shown in Figure 5a, the d-spacing of organo-MMT was determined to be 2.49 nm, indicating an increase from that of MMT-Ca 2+ (1.23 nm). Furthermore, the d-spacing of MMT-G was calculated to be 2.49 nm, which is similar to that of organo-MMT. Notably, the increase in the d-spacing of the nanofiller facilitated even dispersion in the PP matrix. The XRD patterns of GO and CMG + are shown in Figure 5b, revealing graphene interlayer distances of 9.601 and 3.759 Å for GO and CMG + , respectively. Because graphene is obtained through the exfoliation of graphite, they have identical crystal structures, suggesting a layer spacing of 3.36 Å . Further, the layer spacing of GO increased owing to oxidation whereas that of CMG + narrowed again because of the small number of peripheral defects as a result of reduction and functionalization. In particular, the layer spacing of CMG + was wider than that of graphite because of the introduction of functional groups, and the dif fraction peak of CMG + was also broad.  Figure 6 shows images of the water contact angle measurements of MMT-Ca 2+ , GO CMG + , organo-MMT, MMT-G nanofillers with various graphene contents, and the PP/MMT-G nanocomposite. The surface of MMT-Ca 2+ is hydrophilic because of its structure (as discussed earlier), and as illustrated in Figure 6, owing to the hydrophilic properties of MMT-Ca 2+ , a substantially low contact angle (12.0°) was obtained. As previously  Figure 6 shows images of the water contact angle measurements of MMT-Ca 2+ , GO, CMG + , organo-MMT, MMT-G nanofillers with various graphene contents, and the PP/MMT-G nanocomposite. The surface of MMT-Ca 2+ is hydrophilic because of its structure (as discussed earlier), and as illustrated in Figure 6, owing to the hydrophilic properties of MMT-Ca 2+ , a substantially low contact angle (12.0 • ) was obtained. As previously reported by our research group [17], the surface of GO is hydrophilic because of the hydroxyl and carboxyl groups and exhibits a low water contact angle (53.5 • ). However, CMG + is hydrophobic, which can be attributed to the reduction in the number of hydrophilic functional groups (hydroxyl and carboxyl groups) and the increase in the crystallinity of graphene.
MMT increased, the water contact angle correspondingly increased, in addition to an increase in hydrophobicity. However, a higher graphene content results in a higher cost of the PP nanocomposite. Therefore, the determination of the optimal content is necessary. The 1.0 wt% graphene sample (PP/MMT-1.0G nanocomposite, contact angle of 101.4°) and 3.0 wt% graphene sample (PP/MMT-3.0G nanocomposite, contact angle of 101.7°) had similar contact angles. Therefore, in this study, we prepared PP nanocomposites with a graphene content of 1.0 wt% (MMT-1.0G).  Figure 7 shows the mechanical properties of the PP nanocomposites in terms of the MMT-G content. As described earlier, MMT-1.0G (graphene content of 1.0 wt%) was used, and the standard used for mechanical property evaluation was commercial PP-30 wt% talc composite (data are shown by the dashed line in Figure 7).

Nanocomposite Characterization
As shown in Figure 7, the mechanical properties of the PP/MMT-G nanocomposites improved with the addition of the nanofillers, probably because of their surface-modifying effects that resulted in improved compatibility with the PP matrix via enhanced mechanical interlocking/adhesion at the nanofiller-matrix interface [26]. In general, the mechanical properties of polymeric nanocomposite materials depend on the size of particles, degree of dispersion, polymer-filler interfacial properties, and filler content [20]. The infiltration of the nanofiller also has a significant effect on the viscosity and melt flow of the composites and subsequent part processing [27]. As shown in Figure 7a, the melt flow index decreases with an increase in the nanofiller content, as frequently observed for nanocomposites. As expected, the tensile properties of the nanocomposites improved owing to the high rigidity of the nanofillers. Moreover, all physical properties of the PP/MMT-G nanocomposites were superior to those of the commercial PP-30 wt% talc composite. However, the specific gravity (1.10 g cm −1 ) of the PP/MMT-G nanocomposite In contrast to that of MMT-Ca 2+ , the water contact angle of the organo-MMT surface treated with DDAC was significantly improved. Moreover, as the amount of graphene in MMT increased, the water contact angle correspondingly increased, in addition to an increase in hydrophobicity. However, a higher graphene content results in a higher cost of the PP nanocomposite. Therefore, the determination of the optimal content is necessary. The 1.0 wt% graphene sample (PP/MMT-1.0G nanocomposite, contact angle of 101.4 • ) and 3.0 wt% graphene sample (PP/MMT-3.0G nanocomposite, contact angle of 101.7 • ) had similar contact angles. Therefore, in this study, we prepared PP nanocomposites with a graphene content of 1.0 wt% (MMT-1.0G). Figure 7 shows the mechanical properties of the PP nanocomposites in terms of the MMT-G content. As described earlier, MMT-1.0G (graphene content of 1.0 wt%) was used, and the standard used for mechanical property evaluation was commercial PP-30 wt% talc composite (data are shown by the dashed line in Figure 7).

Nanocomposite Characterization
As shown in Figure 7, the mechanical properties of the PP/MMT-G nanocomposites improved with the addition of the nanofillers, probably because of their surface-modifying effects that resulted in improved compatibility with the PP matrix via enhanced mechanical interlocking/adhesion at the nanofiller-matrix interface [26]. In general, the mechanical properties of polymeric nanocomposite materials depend on the size of particles, degree of dispersion, polymer-filler interfacial properties, and filler content [20]. The infiltration of the nanofiller also has a significant effect on the viscosity and melt flow of the composites and subsequent part processing [27]. As shown in Figure 7a, the melt flow index decreases with an increase in the nanofiller content, as frequently observed for nanocomposites. As expected, the tensile properties of the nanocomposites improved owing to the high rigidity of the nanofillers. Moreover, all physical properties of the PP/MMT-G nanocomposites were superior to those of the commercial PP-30 wt% talc composite. However, the specific gravity (1.10 g cm −1 ) of the PP/MMT-G nanocomposite (0.5 wt% in this study) was lower than that of the commercial PP-30 wt% talc composite (1.14 g cm −1 ). This should contribute to weight reduction when applied in vehicles. Figure 8 shows the M max (maximum moisture absorption) of the PP nanocomposites with respect to the nanofiller content. Because of the hydrophobic properties of MMT-G and the interruption to the water diffusion path, the moisture absorption decreased with the increase in the MMT-G content. Specifically, for the PP/MMT-G nanocomposites, the moisture absorption decreased at low filler contents and then increased rapidly at significantly high filler contents (i.e., higher than 1.0 wt% MMT-G). This is because although the hydrophilicity of MMT is suppressed by the graphene coating, the hydrophilicity ratio (silicate ratio) increases above a certain level. Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 12 (0.5 wt% in this study) was lower than that of the commercial PP-30 wt% talc composite (1.14 g cm −1 ). This should contribute to weight reduction when applied in vehicles.  Figure 8 shows the Mmax (maximum moisture absorption) of the PP nanocomposites with respect to the nanofiller content. Because of the hydrophobic properties of MMT-G and the interruption to the water diffusion path, the moisture absorption decreased with the increase in the MMT-G content. Specifically, for the PP/MMT-G nanocomposites, the moisture absorption decreased at low filler contents and then increased rapidly at significantly high filler contents (i.e., higher than 1.0 wt% MMT-G). This is because although the hydrophilicity of MMT is suppressed by the graphene coating, the hydrophilicity ratio (silicate ratio) increases above a certain level.  (0.5 wt% in this study) was lower than that of the commercial PP-30 wt% talc composite (1.14 g cm −1 ). This should contribute to weight reduction when applied in vehicles.  Figure 8 shows the Mmax (maximum moisture absorption) of the PP nanocomposites with respect to the nanofiller content. Because of the hydrophobic properties of MMT-G and the interruption to the water diffusion path, the moisture absorption decreased with the increase in the MMT-G content. Specifically, for the PP/MMT-G nanocomposites, the moisture absorption decreased at low filler contents and then increased rapidly at significantly high filler contents (i.e., higher than 1.0 wt% MMT-G). This is because although the hydrophilicity of MMT is suppressed by the graphene coating, the hydrophilicity ratio (silicate ratio) increases above a certain level.   Figure 9 shows images of headlamp housings prepared using the conventional PP composite and the nanocomposite prepared in this study. No significant differences were observed in processability or product shape between the materials. Further, because of the low specific gravity of the PP/MMT-G 0.5 wt% nanocomposite, the housing weight was 877.5 g, whereas the weight of the housing prepared using the commercial PP composite was 910.0 g. Therefore, the weight reduction rate was 3.6%. Figure 9 shows images of headlamp housings prepared using the conventional PP composite and the nanocomposite prepared in this study. No significant differences were observed in processability or product shape between the materials. Further, because of the low specific gravity of the PP/MMT-G 0.5 wt% nanocomposite, the housing weight was 877.5 g, whereas the weight of the housing prepared using the commercial PP composite was 910.0 g. Therefore, the weight reduction rate was 3.6%. Figure 9. Digital photographs (a, b) and shape deviation color maps (a-1, b-1) used to evaluate the production accuracy of the headlamp housings prepared using (a, a-1) commercial PP composite and (b, b-1) PP/MMT-G nanocomposite.

Application as a Headlamp Housing
In the preparation of headlamp housings, several factors influence the dimensional accuracy of the design [28]. Therefore, for forward engineering, in the initial design, dimensional tolerances are considered to reflect variations in the manufacturing process. In particular, polymer composites exhibit different degrees of shrinkage or twisting after injection depending on the shape or type of the additive. Therefore, the dimensional accuracy of the prepared parts was determined using coordinated measurements using a 3D measuring instrument after injection manufacturing. Specifically, a 3D image was obtained by measuring along a predetermined path to determine deviations in the dimensions of the produced parts from those of the computer-aided design (CAD) model. Figure  9a-1,b-1 show the shape deviation color map derived for the headlamp housings prepared using the conventional composite and nanocomposite, respectively. Notably, the measured maximum deviation is 0.38 mm for the commercial PP composite and 0.67 mm for the PP/MMT-G nanocomposites. Therefore, the dimensional accuracy of the PP/MMT-G nanocomposite is higher than that of the conventional PP composite. Figure 9. Digital photographs (a,b) and shape deviation color maps (a-1,b-1) used to evaluate the production accuracy of the headlamp housings prepared using (a,a-1) commercial PP composite and (b,b-1) PP/MMT-G nanocomposite.
In the preparation of headlamp housings, several factors influence the dimensional accuracy of the design [28]. Therefore, for forward engineering, in the initial design, dimensional tolerances are considered to reflect variations in the manufacturing process. In particular, polymer composites exhibit different degrees of shrinkage or twisting after injection depending on the shape or type of the additive. Therefore, the dimensional accuracy of the prepared parts was determined using coordinated measurements using a 3D measuring instrument after injection manufacturing. Specifically, a 3D image was obtained by measuring along a predetermined path to determine deviations in the dimensions of the produced parts from those of the computer-aided design (CAD) model. Figure 9a-1,b-1 show the shape deviation color map derived for the headlamp housings prepared using the conventional composite and nanocomposite, respectively. Notably, the measured maximum deviation is 0.38 mm for the commercial PP composite and 0.67 mm for the PP/MMT-G nanocomposites. Therefore, the dimensional accuracy of the PP/MMT-G nanocomposite is higher than that of the conventional PP composite.

Conclusions
In this study, graphene and MMT-based nanofillers were developed to reduce the moisture absorption of the PP composites used to prepare automotive headlamp housings. As a result of the hydrophilicity control facilitated by the use of the nanofiller, the moisture absorption of the PP nanocomposites was lowered by up to 11.22% (i.e., for the MMT-G 0.5 wt% nanocomposite). Overall, considering the moisture absorption, mechanical properties, and cost, the PP/MMT-G 0.5 wt% nanocomposite showed the optimal performance. In particular, the PP/MMT-G 1.0 wt% nanocomposite showed the lowest maximum moisture absorption among the analyzed samples. However, this composite is unsuitable for industrial applications because of its poor mechanical properties. Nevertheless, by manufacturing a headlamp housing with the derived composition, a weight reduction of 32.5 g (3.6%) compared to that of the product prepared using the commercial PP-30 wt% talc composite was achieved. Therefore, the designed nanocomposites are expected to help mitigate headlamp fogging while reducing weight.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.